Re: [Qemu-devel] [v5][PATCH 0/5] xen: add Intel IGD passthrough support

[Chen, Tiejun]

On 2014/6/25 17:54, Paolo Bonzini wrote:
> Il 25/06/2014 11:50, Chen, Tiejun ha scritto:
> > > For past devices, we know which BARs they use.  For future devices, it
> > > would be nice if the PCH/MCH backdoor was specified so that we know they
> > > will leave a free BAR for virtualization.
> >
> > Now I'm a bit confused about BAR here.
> >
> > You're saying we will reserve a free BAR to address those information to
> > expose to guest, but which device does this free BAR belong to? The
> > video device? Or PCH/MCH?
>
> The video device.  If the host device does not have the BAR (which will
> be the common case), QEMU can emulate it like this:

According to some feedback, neither we have any unused PCI unused config 
offset, nor BAR.

> - offsets 0x0000..0x0fff map to configuration space of the host MCH

Are you saying the config space in the video device? but will this 
overlap? Every PCIe device already have a 4K config space, right? So we 
should extend these two ranges:

0x0000..0x0fff: the standard PCIe config space in the video device
0x1000..0x1fff: map to configuration of the real host bridge
0x2000..0x2fff: map to configuration of the real ISA bridge

Right?

But as you know, we just need to expose a little config space from the 
real host bridge and the real ISA bridge, so this may be waste with 8K.

Thanks
Tiejun

> - offsets 0x1000..0x1fff map to configuration space of the host PCH
>
> Of course this is only limited to offsets that are needed by the driver.
>
> Paolo